I've been wanting to upgrade Tauro into a 10gallon community tank for a while now. Right now he lives in a 4 gallon with his pet mystery snail. The only problem is I'm too scared to bring in any new fish in case they get him sick. I had a bad experience when I bought 2 mystery snails for Tauro and his neighbour Dante, but Dante's snail apparently brought something with it and Dante passed away. I didn't even know how to QT a snail :/
Anyway, I'm pretty inexperienced with aquariums so I'm wondering if people could give me tips on :

- what fish I can keep in a 10g with Tauro (not ghost shrimp, they are hard to get where I live & my LFS's cherry shrimp are way too small for Tauro)

- how I would go about QTing the newcomers?

- do I need to cycle a 10 gallon? I have an "Aqua One Clearview 100" filter but I've never used it because it's too strong for Tauro's 4gallon. Any quick links/tips for cycling a 10g? :)

- is it better/easier to have a planted tank? I only have plastic plants in Tauro's current tank. There is duckweed floating, but the snail eats it constantly. What if the plants bring in disease, can I QT plants?

Yes, you need to cycle a 10 gallon. I've tried a few times to "speed cycle" a tank using various methods and chemicals, none have worked for me, so I would recommend setting up your 10 gallon, adding dechlorinator, a heater, a filter, and aerator, and many many plants! Depending on what plants you want, you may want to look into plant-specific substrates to put under your gravel so your plants flourish more, as well as good lighting. Invest in a liquid test kit, and monitor your levels. Here's a thread on how to cycle: http://www.bettafish.com/showthread.php?t=47838
If you use the filter you have and it's still too strong, you can rubberband a sponge on the outflow of the filter and it will slow the water movement.
I believe plants are a wonderful addition to a 10 gallon, they help keep my ammonia at bay, but it makes cleaning a little difficult. As far as I know, plants don't carry disease (I could be wrong), but they often carry snails. It starts with one... then 10, then suddenly you're at 50 snails. So be sure to rinse your plants a LOT before putting them in your tank, and if you do get a snail, carefully siphon it up and out of your tank. DO NOT SQUISH IT, it will release eggs that way. :P
Once your tank is cycled, you can start a community tank one of two ways, it depends on the temperament of your betta. Most say add the betta last, so he feels like the newbie, and doesn't feel the need to "defend" his territory. I had a super docile and relaxed betta. He was first and I later added tank mates. He could care less. Test the temperament of your betta with a mirror, does he flare and want to fight? Or does he not give a darn skippie like mine? If he does flare, consider adding him last, or your have to be selective on what types of tank mates you have. I have African Dwarf Frogs and a Cory catfish. The wonderful thing about those is that they chill at the bottom, are not colorful, and do not look moderately CLOSE to another betta. Some bettas hate colorful mollys/platys because they resemble bettas and are a threat. With frogs, the betta may be like "what... what is that thing? He's ugly, the ladies won't want him. No need to worry" haha.
If you add your betta first, it makes QT newcomers easier, as you have your bettas old tank to put your newcomers in and monitor for a while. Ask your local fish store when they get their shipments in, and don't shop for a tank mate 3-4 days until after the shipment, that way any sick ones or duds will die off, and you're left with the (hopefully) healthy batch.

So some options on tank mates:
African Dwarf Frogs (need around 3, they are community)
Mystery Snail
Ghost Shrimp (I know you said you can't get them, but maybe in future)
Pygmy Cory Catfish (need at least 3, they are community)

I'll attach a photo of my 10 gal. My betta isn't in the photo because he was in a hospital tank at the time, but he's normally in there. When you add tank mates, be sure there are plenty of spaces to hide, so if there is some tension, they can go their separate ways to their little special place and relax.
Best of luck!